Gas field ion sources are known. Typically, a gas field ion source includes a tip formed of electrically conductive material. During use, the tip can be used to ionize neutral gas species to generate ions by bringing the neutral gas species into the vicinity of the tip while applying a high positive potential to the apex of the tip. The ions can, for example, be formed into a beam that can be used to determine certain properties of a sample and/or to modify the sample.